Air condenser installations are composed of air condensers in which steam is condensed by fan supplied air. The air condensers consist of inclined or upright finned cooling tubes connected, at their upper ends, to a common steam distribution chamber or manifold and, at their lower ends, to a common condensate collecting chamber or collection header. The inclined or upright position of the cooling tubes ensures that the condensate of the steam reaches the collecting chamber by means of gravity.
In operation, the exhaust steam introduced from the steam distribution chamber into the cooling tubes becomes, while flowing, condensed at constant temperature. Resulting liquid drops form a liquid film flowing downwards on the inner walls of the cooling tubes. The temperature of the liquid film is substantially equal to the temperature of the incoming exhaust steam.
If the length of the cooling tubes is such that condensation is terminated just at the end of the cooling tubes, the temperature of the condensate withdrawing into the collecting chamber is substantially likewise equal to the temperature of the entering exhaust steam.
If, on the other hand, due to changes of operational factors such as load, temperature of cooling air and the like, the length of the cooling tubes becomes smaller than the previously specified value, condensation of the exhaust steam will not be finished within the cooling tubes which, then, discharge steam as well. This amount of steam which is assessed to 20 percent of the total steam flow, is condensed in a secondary condensation zone. Structure of the air condensers of the secondary condensation zone is the same as that of the already described primary condensation zone. However, an operational difference consists in that the steam flows, for reasons irrelevant here, as a rule, in upward direction in their cooling tubes.
Should the length of the cooling tubes surpass the above mentioned value, condensation will be terminated before the condensate reaches the collecting chamber. Then, the liquid film flowing downwards on the inner walls of the cooling tubes is further cooled down so that, dependent on the tube length, considerable differences between the temperatures of the entering exhaust steam and the exiting condensate may appear: the condensate becomes undercooled.
Undercooling of the condensate is undesirable for two reasons.
On the one hand, it represents thermodynamical loss because the undercooled condensate can be reevaporated but by combusting surplus fuel which could be spared if there were no undercooling.
On the other hand, undercooling may provoke danger of frost since the undercooled condensate may freeze at low ambient temperatures and entail frost damages in the air condensers of the installation.